TRH has the structure

The nomenclature of Schechter and Berger is used to describe the positions of the peptide substrate residues (P) relative to the scissile P1—P1′ bond and the corresponding subsets (S) in the active site of the enzyme. In other literature, the right portion of the molecule is called the “prolineamide” or “C-terminal” portion; the centre portion of the molecule is called the “histidyl” portion; and the left portion of the molecule is called the “pyroglutamyl”, or “N-terminal” portion.
Thyrotropin-releasing hormone (TRH) (pyroglutamyl-histidyl-prolineamide, Glp-His-ProNH2) is a naturally occurring neuroactive peptide with multiple actions in the central nervous system (CNS) that have been shown to be beneficial in the treatment of CNS disorders, including brain and spinal injury, stroke, epilepsy and spinocerebellar degeneration. While mechanisms underlying the therapeutic actions of TRH are not fully understood, it is recognized by the art that TRH has substantial beneficial effects due in part to its action in mitigating the secondary neuronal cell damage caused by a sequence of biochemical reactions triggered by the primary injury. This same sequence of reactions has been found to occur in both acute and chronic neurodegeneration and drugs capable of disrupting this sequence have potentially broad application as neuroprotectants. It is becoming evident that those targeting multiple components of the sequence may offer therapeutic advantages over pharmacological interventions targeted at single components. Notably TRH has been shown to antagonize the actions of multiple constituents of the sequence and also improve critical biochemical functions impaired by CNS trauma such as cell bioenergetics. TRH's neurotrophic actions may also be beneficial for restoring loss of function associated with neurodegeneration.
Recent literature highlights a growing recognition of the breadth of TRH functions and the potential widespread clinical applicability of this remarkable peptide. For example, it has been recognized recently that TRH may function as a core homeostatic regulator within four integrated CNS systems and as such may have extensive involvement and therapeutic application in human illnesses associated with disturbances in neurobiological function, including conditions as diverse as jetlag, obesity and depression. Other research strongly indicates a role for TRH in the physiology and treatment of mood disorders and epilepsy. In a recent paper (Luo et al., 2002) it was suggested that TRH might function as an endogenous neuroprotectant and that the low levels of TRH found in the hippocampus of Alzheimer's patients may possibly contribute to the pathogenesis of this disorder. In relation to epilepsy, the art shows that TRH is efficacious in treating patients with intractable epilepsy. In addition, the art indicates that the brain naturally releases TRH in response to seizures and that endogenous TRH has an anticonvulsant function in brain and plays a homeostatic role in reducing the potentially damaging effects of epilepsy.
The art shows that in human clinical trials TRH has a large therapeutic window and is well tolerated. The clinical utility of TRH is, however, severely limited by its susceptibility to enzymic degradation, which significantly reduces TRH bioavailability and duration of action (Kelly, 1995). This is reflected in a disappointing performance produced by native TRH in clinical trials. The short half-life of TRH, arising from enzymic degradation, is also a serious impediment to further investigation of the functions of TRH.
As a means to overcome the susceptibility of TRH to metabolism the art has, to date, concentrated largely on the development of improved delivery systems and degradation-stabilized TRH analogs, which target TRH receptors and act as TRH mimetics. U.S. Pat. No. 5,244,884 relates to thionated analogs of TRH type compounds, which selectively bind to TRH receptor binding sites in animals with high affinity and have potentially pharmacological advantages over TRH in treating pathological conditions in which the effects of TRH have been shown to be beneficial. U.S. Pat. Nos. 5,112,804, 5,428,006 5,693,608, 5,804,212, 6,491,939, 6,524,557 describe methods for administrating a therapeutically effective amount of biologically active substances, including TRH. U.S. Pat. No. 5,686,420 describes a series of novel TRH analogs wherein the C-terminal prolineamide moiety has been preserved, the N-terminal moiety comprises one of five different ring structures and the histidyl moiety is substituted with CF3, NO2 or a halogen and use of these analogs in the treatment of neurological disorders. US patent application 20020004062 describes methods and compositions for providing prolonged release of therapeutic agents, including TRH. U.S. Pat. No. 6,475,989 relates to peptides with the general formula Glp-X-ProNH2 and their potential therapeutic application. Thus far, one degradation-stabilized analog has been approved for therapeutic use in humans; this was launched by Tanabe Seiyaku Co., Ltd., in 2000 for the treatment of spinocerebellar degeneration, an orphan drug designated disorder. In contrast to TRH analogs described in U.S. Pat. No. 5,686,420 and US patent application 20020004062, the C-terminal prolineamide moiety that is found in TRH and TRH-like peptides has not been preserved in compounds described in the present invention, described herein.
The art shows that many degradation-stabilised analogs that act as TRH mimetics contain modifications to the N-terminal pyroglutamyl residue of TRH (Kelly 1995, Horita 1998, Faden & Salzman 1992). For example, the N-terminal pyroglutamyl residue of TRH has been replaced by a 6-membered ring (e.g. TA-0910 i.e. Ceredist, CG3509, CG3703), a different 5-membered ring (e.g. DN1417, JTP2942) and a 4-membered ring (e.g. YM14673). Such analogs have been shown by the art to mimic the central actions of TRH, despite displaying reduced affinity for TRH receptors.
The following are a selection of known TRH analogs:

An alternative approach to facilitate the clinical use of TRH is to protect it from degradation by inhibiting its enzymic degradation. TRH-degrading ectoenzyme (TRH-DE) (EC 3.4.19.6), also known as pyroglutamyl aminopeptidase II (PAP-II, PP-II) is recognized by the art to be the enzyme responsible for degrading extracellular TRH. Thus, TRH-DE is an attractive therapeutic target because of the potential offered by TRH-DE inhibitors to enhance the therapeutic effects of TRH. TRH-DE catalyzes the removal of the N-terminal pyroglutamyl group from TRH and is located on the surface of neuronal cells. A soluble form of the enzyme, known as thryoliberinase, is present in serum. TRH-DE appears to be a rare example of a neuropeptide-specific peptidase in that it displays absolute functional specificity for TRH. Furthermore, TRH is not degraded by any other enzymes that are in a position to affect TRH signaling. Hence, the modulation of TRH-DE activity should, in principle, amplify TRH effects exclusively. The special relationship between TRH-DE and TRH should ensure that only the biological actions of TRH are amplified and thus, lead to minimal side effects.
To date, there is no crystal structure or homology model for TRH-DE on which to base the rational design of active site directed inhibitors. Nevertheless, through research carried out by the inventor named herein novel, potent. competitive inhibitors of TRH-DE have been identified (Kelly et al. 2000a, 2000b, U.S. Pat. App. 20030166944). Prior to this, few TRH-DE inhibitors had been reported—the most potent of these was N-[1-carboxy-2-phenylethyl]N-imidazole benzyl histidyl-β-naphthylamide (Ki of 8 mM) (Charli et al., 1989). A recent paper by Pascual et al., (2004) has described the isolation of a TRH-DE inhibitory activity from a marine invertebrate. However, the type of inhibition has not been defined, its molecular structure is not yet known, nor is it certain that inhibition is due to a single chemical entity.
The art indicates that actions of peptides, such as TRH, are mediated by specific receptors. Two TRH receptor subtypes have been described thus far: TRH receptor 1 (TRHR1) and TRH receptor 2 (TRHR2). These receptors have distinct amino acid sequences and distribution patterns, but both display a similar high-affinity for [3H][3-Me-His2]TRH. TRHR1 is highly conserved between species, including human. Unlike TRHR1, TRHR2 has not been identified in humans. In rat TRHR2 expression is restricted to the CNS. This contrasts with TRHR1, which displays very limited mRNA expression in the CNS. The regional distribution of TRHR2 mRNA has been found to be consistent with the possibility that this TRH receptor is involved in mediating the higher cognitive functions of TRH, as well as its effects on arousal, locomotor activity and pain perception. TRHR1 distribution on the other hand indicates that this receptor is involved in mediating the endocrine functions of TRH. Replacement or modification of the central histidyl residue of TRH has been shown by the art to severely reduce receptor affinity (Gershengorn and Osman, 1996). Only one exception has been identified that binds with greater affinity than TRH to TRH receptors and that is [3-Me-His2]TRH. Degradation stabilized analogs described in U.S. Pat. Nos. 4,906,614, 5,244,884 and 5,686,420 have all been found to bind to TRH receptor sites within the brain, albeit with less potency than TRH. The art indicates that the structural preferences for ligand binding to TRH receptors and TRH-DE are different (Kelly et al., 2002). For example, both Glp-Asn-ProAMC and Glp-Asn-ProNH2 have been shown to be potent inhibitors of TRH-DE, but display low affinity compared to TRH for [3H][3-Me-His2]TRH-labeled receptors in rat cortical membranes.
The present invention describes novel compounds that potently inhibit TRH-DE and or bind to TRH receptors with high affinity and substantially enhance TRH actions in rat. Potent TRH-DE inhibitors that also bind to TRH receptors and amplify TRH effects have not been previously described and are first in their class. These compounds are distinct from those described in U.S. Pat. Application 20030166944 because they inhibit TRH-DE and or bind to TRH receptor(s). Further, unlike the compounds disclosed herein, the amino acids contained within the structure of TRH-DE inhibitors described in U.S. Pat. Application 20030166944 are all in the L-configuration. Certain compounds of the invention are also different from other compounds that have been shown to bind to TRH receptors with the exception of 3-Me-His2TRH, in that they display greater affinity for TRH receptors than TRH. 3-Me-His2TRH is the only compound described thus far that also binds to TRH receptors with greater affinity than TRH.